In many emerging technologies, such as electric vehicles and hybrids thereof, there exists a need for capacitors with both high energy and high power densities. Much research has been devoted to this area, but for many practical applications such as hybrid electric vehicles, fuel cell powered vehicles, and electricity microgrids, the current technology is marginal or unacceptable in performance and too high in cost. See DOE Progress Report for Energy Storage Research and Development fy2005 (January 2006) and Utility Scale Electricity Storage by Gyuk, manager of the Energy Storage Research Program, DOE (speaker 4, slides 13-15, Advanced Capacitors World Summit 2006).
Electrochemical double layer capacitors (EDLC's, a form of electrochemical capacitor called an ultracapacitor, sometimes also called a supercapacitor) are one type of capacitor technology that has been studied for such applications. Electrochemical double layer capacitor designs rely on very large electrode surface areas, which are usually made from “nanoscale rough” metal oxides or activated carbons coated on a current collector made of a good conductor such as aluminum or copper foil, to store charge by the physical separation of ions from a conducting electrolyte into a region known as the Helmholtz layer that forms immediately adjacent to the electrode surface. See U.S. Pat. No. 3,288,641. There is no distinct physical dielectric in an EDLC. Nonetheless, capacitance is still based on physical charge separation across an electric field. The electrodes on each side of the cell and separated by a porous membrane store identical but opposite ionic charges at their surfaces within the double layer, with the electrolyte solution in effect becoming the opposite plate of a conventional capacitor for both electrodes.
It is generally accepted that EDLC internal carbon pore size should be at least about 2 nm for an aqueous electrolyte or at least about 3 nm for an organic electrolyte to accommodate the solvation spheres of the respective electrolyte ions in order for the pores to contribute their surface for Helmholtz double layer capacitance. See J. Electrochem. Soc. 148 (8): A910-A914 (2001) and Electrochem. and Solid State Letters 8 (7): A357-A360 (2005). Internal pores also should be accessible from the outer particle surface for electrolyte exposure and wetting, rather than sieving. The more total accessible surface, the better. Conventional activated carbons used in ELDC devices have many electrochemically useless micropores (i.e., below 2 nm according to the IUPAC definition). In the highly activated electrochemical carbons reported in the literature, total utilized surface is typically thought to be 10% (see U.S. Pat. No. 6,491,789) to 20% (see U.S. Pat. No. 6,737,445). A typical mesopore proportion in commercial electrocarbons may range from a low of 5% to a high of 22% mesopore with DFT surfaces ranging from about 1300 m2/g to about 1900 m2/g. See Walmet (MeadWestvaco), Proceedings of the 16th International Seminar on DLC (ISDLC): 139-140 (2006).
Several alternative approaches to producing a high usable surface carbon suitable for EDLC devices using organic electrolytes at their desirable higher operating voltages have been undertaken. These include unusual carbon precursors (for example, U.S. Pat. No. 6,660,583), novel carbonization regimes (for example U.S. Pub. No. 2005/0207961), novel physical activation regimes lasting many hours or even days (for example those mentioned by Norit Nederland B V—a major activated carbon supplier—in Proceedings of 16th ISDLC: 95 (2006) as “steam activation at 800-1000 C” with residence time in the kiln “from several hours to several days”), novel chemical activation regimes (for example, U.S. Pat. No. 5,877,935), carbon aerogels (for example, U.S. Pat. No. 5,626,977, U.S. Pat. No. 5,898,564), various templating techniques (for example U.S. Pat. No. 6,297,293, U.S. Pub. No. 2004/0091415, U.S. Pat. No. 6,737,445), carbide derived carbons (for example, PCT/EE20051000007, U.S. Pub. No. 2006/0165584) and carbon nanotubes (for example, U.S. Pat. No. 6,491,789 and U.S. Pat. No. 6,934,144) or equivalents (for example, U.S. Pub. No. 2005/0025974). Each of these approaches has significant limitations. An application by one of these inventors, PCT/US2007/004182 (claiming priority to U.S. Provisional Application No. 60/773,538, filed Feb. 15, 2006) analyzes these limitations, and discloses a novel method of producing improved mesoporous carbons from any suitable precursor by use of catalytic nanoparticles.
For applications including electrochemical capacitors, hybrid capacitor/battery devices such as Fuji Heavy industries LiC, and asymmetric batteries, it is desirable to precisely engineer the mesoporous carbon to the requirements of the device (energy density, power density, electrolyte system) by suitably adjusting the induced mesoporosity.
While surface area created by internal porosity is important for filtration carbons, the prior art overlooks the effect of exterior surface. True exterior surface is particularly important for specific capacitance because of sieving microporosity. Increasing true exterior surface for any given geometric object by definition means increasing rugosity. Exterior surface of an activated carbon particle or fiber is by definition accessible to electrolyte, whereas a substantial proportion of interior pore surface probabilistically is not available because of sieving micropores. See PCT/US2007/004182. Discussion of the true exterior of activated carbons is virtually non-existent in the electrocarbon scientific literature, since conventionally it was thought to be very small in comparison to the internal pore surface of activated carbons. Two of three notable exceptions suffer from methodological flaws.
First, in Electrochimica Acta 41 (10): 1633-1639 (1996) Shi assumed that all mesopores were external. This is readily proven not true. See, for example, Chem. Mater. 8: 454-462 (1996), J. Electrochem. Soc. 149 (7): A855-A861 (2002), and Electrochem. and Solid State Letters 6 (10): A214-A217 (2003). Moreover, certain results Shi derived using this assumption he himself could not fully explain. Extrapolations therefrom can be problematic. See Carbon 43: 1303-1310 (2005).
Second, in J. Power Sources 154: 314-320 (2006) Stöckli attempted to reinterpret Shi using multivariate statistical analysis on a different sample set of activated carbons, with partial success. Stöckli assumed that external surface measured using the Brunauer, Emmett and Teller method (BET) was relative to that of a “nonporous” carbon black. However, it has been shown that while the BET surface of reference Vulcan XC 72 carbon black is about 254 m2/g (manufacturer spec), ranging from 259 m2/g BET in J. Power Sources 154: 314-320 (2006) to 240 m2/g BET in Carbon 43:1303-1310 (2005), the surface measured by Density Functional Theory (DFT) in those papers is only 132 m2/g and 119 m2/g respectively. The theoretical geometric surface of a random packing of perfect hard identical spheres (using the Bernal packing limit 0.635 for the manufacturer Vulcan XC 72 specification of D50 30 nm and 1.80 g/cc) is 111 m2/g-reasonably close to the DFT estimate. Therefore the BET carbon black surface produces errors by a factor of about 2. Without wishing to be limited by theory, this is likely due to capillary condensation in the mesopore voids between the nanospheres. See Condon, Surface Area and Porosity Determination by Physiosorption, (1998) pp. 160-169, and Adsorption 4:187-196 (1998). Therefore, comparative BET external surface estimates are suspect. Stöckli's statistically derived external surface contribution coefficients are only 0.124 (equation 5) to 0.134 (equation 10) for the smaller exterior, versus higher contribution coefficients of at least 17.7 (equation 5) to 17.4 (equation 10) for the much larger interior surface. In effect, the suspect external (under) estimates resulted in multivariate equations suggesting most capacitance arises from internal surfaces in accordance with conventional thinking.
Third, in Proceedings of 16th ISDLC: (special late session available in the electronic version, pages 18-20) (2006), Ikeda of Asahi Research Labs used 19F nuclear magnetic resonance (NMR) of the anion BF4 in propylene carbonate to characterize the carbon location of adsorbed anions in a working EDLC device. This was part of a study of additives for improving low temperature conductivity and the breakdown withstand voltage of propylene carbonate solvent. See U.S. Pat. No. 6,879,482. The experiments show convincingly that, depending on the probability of access to interior pore surface (indicated by increasing average pore size), from none to only about half of the anions (at very large average pore sizes greater than 1.5 nm) were internally adsorbed. The anion is the smaller of the two in the tested electrolyte system; the larger cation has less access and is therefore kinetically controlling. See for example Carbon 40: 2613-2626 (2002). At least half of the anions, and therefore more than half of the kinetically controlling cations, were shown by NMR to be adsorbed on the exterior surface. This is a surprisingly high value compared to conventional thinking, but fully consistent with the disclosures in PCT/US2007/004182. It is direct experimental evidence of the importance of the external surface of activated carbons.
True exterior surface originates more than half of total specific capacitance in typical activated carbons. Increasing exterior surface by increasing rugosity therefore is important for improving the energy density of electrocarbons.
As a direct consequence of the importance of exterior surface, it follows directly from Euclidean geometry that smaller particles have more exterior for a given volume/mass of carbon. As an example of the utility of smaller particles, carbide derived carbons (CDC's) have specific capacitance as high as 120-135 F/g despite having almost entirely microporous pore distributions. CDC's typically have a tight particle size distribution from about 1-3 microns (see for example J. Power Sources 133: 320-328 (2004) and J. Power Sources 162: 1460-1466 (2006)) to about 6 microns (see for example, Electrochem. and Solid State Letters 8 (7): A357-A360 (2005)). They consequently have among the highest volumetric energy densities of all carbons (see for example, Proceedings of 15th ISDLC: 250 and 259 (2005)). By comparison, commercial electrocarbons typically have a looser D50 of about 8 microns, up to four times larger median particles and a larger overall polydispersion, with proportionately less exterior per unit volume/mass. (See for example, U.S. Pat. No. 6,643,119, and Proceedings of 16th ISDLC: 141 (2006)).
The chosen carbon activation method can impact rugosity, and thus, the specific capacitance of the resulting material. Alkali activation is conventionally found to produce better specific capacitance despite the fact that pore size distributions and average pore sizes of resulting materials are not substantially different than for other activation processes. For example, U.S. Pat. No. 5,877,935 describes material in which at least 40% of the pores are between 1 and 2 nm, i.e. microporous. For detailed analyses of large samples of activated carbons, see for example Electrochemica Acta 41 (10): 1633-1639 (1996), J. Power Sources 74: 99-107 (1998), and J. Power Sources 154: 314-320 (2006). Without wishing to be bound by theory, one mechanism for enhanced usable surface from alkali activation (despite the predominance of micropores) is increased rugosity. See for example, FIG. 1 of Carbon 40: 2613-2626 (2002), and FIG. 2 with accompanying discussion on page E201 of J. Electrochem. Soc. 151 (6): E199-E205 (2004). Based on the micron magnification scales of these imaged activated carbon particles, at least some alkali activation rugosity is at relatively large physical scales on the order of hundreds of nanometers. Visual differences in rugosity with different activation times and at different activation temperatures correlate with observed variations in specific capacitance. Neither mesopore surface, nor mesopore volume, nor total surface, nor increase in average pore size correlated to the observed changes in specific capacitance.
Conventional physical activation also produces rugosity, although principally on a different physical scale. Spalled individual carbon subunits, or small aggregates thereof below 100 nm in dimension, were imaged by DOE project DE-FG-26-03NT41796 (Lehigh University June 2005). Equally important is molecular level etching of individual carbon subunits (without, for example, formation of potassium carbonate particles as in alkali activation) caused by steam or carbon dioxide. This results in oxidative pitting of the carbon exterior surface on a scale of 1 nm to about 10-15 nm, or up to about the dimension of individual carbon subunits. See for example Critical Reviews in Solid State and Mat. Sci. 30: 235-253 (2005). Such subunit scale activated carbon surface pitting has been imaged using high-resolution transmission electron microscopy (HRTEM). See, for example, Economy, Proceedings of the 8th Polymers for Advanced Technology International Symposium, Budapest 11-14 September 2005. Similar oxidative pitting on the same dimensional scale has been directly measured using atomic force microscopy (AFM) on surface treated vapor grown carbon fiber (VGCF). Treatment increases rugosity more than 17 times over untreated VGCF. See Carbon 37 (11): 1809-1816 (1999).
Researchers have previously attempted to combine activation processes without success. For example, Carbon 45 (6): 1226-1233 (2007) reports a new attempt to improve RP-20 (formerly designated BP-20) mesoporosity, a standard electrocarbon (see, for example, U.S. Pat. No. 6,643,119) produced by Kuraray in Japan (hereinafter the “Tartu Paper”). The Tartu Paper discusses a second activation with steam at high temperatures ranging from 950° C. to 1150° C. for 2.5 hours. At 1050° C., this second activation nearly doubled total pore volume, more than doubled mesoporosity, and increased BET surface by a reported 65%. Each of these metrics is conventionally desirable, and the increases are large. But the disappointing result with working two-electrode EDLC devices, given in Table 2 and in FIG. 7 of the Tartu Paper, was that no significant improvement in carbon specific capacitance over RP-20 alone was seen.
Another previous effort (also focused on mesoporosity rather than rugosity) that combined chemical and physical activation together in one step did not produce a better material. A Taiwanese research group published results of simultaneous chemical KOH activation and physical activation with carbon dioxide. In the Journal of Power Sources 159 (2): 1532-1542 (2006) (hereinafter “JPS 2006”) the results on fir wood char were described. In Electrochimica Acta 52 (7): 2498-2505 (2007) (hereinafter “EA 2007”) the results on pistachio nutshell char were described. In both papers, unactivated but carbonized char was mixed with an equal weight (1:1) of KOH, then activated for 60 minutes at 780° C. under inert nitrogen. Conventionally, KOH activation under inert N2 is done on previously carbonized material as described by U.S. Pat. No. 5,877,935 and U.S. Pat. No. 7,214,646. At intervals of the 60-minute processing time, ranging from immediately to never, carbon dioxide was also introduced. This procedure resulted in materials that were only KOH activated for 60 minutes, and materials that also had simultaneous carbon dioxide activation for periods of 15, 30, or 60 minutes. The stated purpose of this procedure in both papers was to increase the proportion of mesopores, since “KOH activation is mainly used to etch char to form micropores” and “Vmeso is directly proportional to the CO2 gasification time.” The highest reported specific capacitance for this combined activation in either paper was 197 F/g using sulfuric acid as the electrolyte. By way of comparison, the commercial specification for simple standard Kuraray RP (BP) 15 is a specific capacitance of 236 F/g in sulfuric acid. Both papers also found an unusual and disappointing result that capacitance actually decreased with increasing CO2 time despite substantial increases in total BET surface and mesoporosity. JPS 2006 expressly considered the results to be an abnormal phenomenon and attributed causality to redox pseudocapacitance. EA 2007 contained a similar discussion. Both papers go on to discuss the potential usefulness of longer duration simultaneous CO2 activation in partial removal of these supposed surface functional groups, since they may increase resistivity and may result in undesirably high self-discharge. However, one of skill in the art would recognize that the constant current charge discharge plots of JPS 2006 FIG. 6 are linear (except very near maximum voltage where the cyclic voltammograms also show a redox peak attributable to electrolyte breakdown) so that little redox pseudocapacitance is involved. Rather, the decline can be attributed to deterioration in surface rugosity with increasing duration of concurrent activation. Similar declines can be inferred for increasing KOH activation alone. See Carbon 40: 2616-2626 (2002) and J. Electrochem. Soc. 151: E199-E205 (2004).
Another carbon consideration is chemical purity. In certain critical filtration applications, for example pharmaceuticals, carbon impurities may undesirably leach into the filtrate. Therefore, it is common that activated carbons are sold in at least two grades, As Activated, and Acid Washed to remove impurities. It is conventionally thought that residual metal ion impurities may undesirably introduce faradic electrochemical redox shuttles, thereby increasing self-discharge and reducing stored charge over time. See, for example, Fujino et. al. in Proceedings of the 15th International Seminar on Double Layer Capacitors pp. 79-80 (2005). A conventional means to remove such electrochemical impurities is acid washing. See, for example, J. Electrochem. Soc. 149 (7) A855-A861 (2002).
Another option is to use purified precursors. See, for example U.S. Pub. No. 2005/0207962 (for purified carbohydrate precursors, e.g. refined sucrose), or U.S. Pat. No. 6,660,583 (for a chemically refined synthetic petroleum pitch precursor). Refined precursors such as sugars or petroleum derivatives are often obtained in bulk form. Smaller particles increase relative exterior surface. In order to achieve smaller, usable particle sizes, the bulk material must be milled to a desired final particle size distribution. This is conventionally done after activation for material handling convenience and cost reasons. See, for example, U.S. Pat. No. 7,214,646. This practice results in particles having exterior cleavage surfaces (milled fragments of the former larger activated particles) with less rugosity than an activated exterior surface. For high-resolution surface image comparisons, see, for example, Economy, Proceedings of the 8th Polymers for Advanced Technology International Symposium, Budapest 11-14 Sep. 2005.